5 research outputs found
An evidence based hypothesis on the existence of two pathways of mitochondrial crista formation
Metabolic function and architecture of mitochondria are intimately linked. More than 60 years ago, cristae were discovered as characteristic elements of mitochondria that harbor the protein complexes of oxidative phosphorylation, but how cristae are formed, remained an open question. Here we present experimental results obtained with yeast that support a novel hypothesis on the existence of two molecular pathways that lead to the generation of lamellar and tubular cristae. Formation of lamellar cristae depends on the mitochondrial fusion machinery through a pathway that is required also for homeostasis of mitochondria and mitochondrial DNA. Tubular cristae are formed via invaginations of the inner boundary membrane by a pathway independent of the fusion machinery. Dimerization of the F1FO-ATP synthase and the presence of the MICOS complex are necessary for both pathways. The proposed hypothesis is suggested to apply also to higher eukaryotes, since the key components are conserved in structure and function throughout evolution
Immuno-electron tomography of ER exit sites reveals the existence of free COPII-coated transport carriers
Transport from the endoplasmic reticulum (ER) to the Golgi
complex requires assembly of the COPII coat complex at
ER exit sites. Recent studies have raised the question as to
whether in mammalian cells COPII coats give rise to COPIIcoated
transport vesicles or instead form ER sub-domains
that collect proteins for transport via non-coated carriers. To
establish whether COPII-coated vesicles do exist in vivo, we
developed approaches to combine quantitative immunogold
labelling (to identify COPII) and three-dimensional electron
tomography (to reconstruct entire membrane structures).
In tomograms of both chemically fixed and high-pressurefrozen
HepG2 cells, immuno-labelled COPII was found on
ER-associated buds as well as on free ~50-nm diameter
vesicles. In addition, we identified a novel type of COPII-coated
structure that consists of partially COPII-coated, 150–200-nm
long, dumb-bell-shaped tubules. Both COPII-coated carriers
also contain the SNARE protein Sec22b, which is necessary
for downstream fusion events. Our studies unambiguously
establish the existence of free, bona fide COPII-coated
transport carriers at the ER–Golgi interface, suggesting that
assembly of COPII coats in vivo can result in vesicle formation